A power conversion device configured to use a semiconductor switching element is such that a voltage surge occurring at either end of the element when switching is a considerable problem. When the semiconductor switching element is, for example, an IGBT, the voltage surge occurs between the collector and emitter (hereafter, this kind of voltage surge will be called a switching surge).
The relationship between a collector-to-emitter voltage Vce occurring when switching the IGBT and a collector current Ic is as shown in, for example, the schematic view of FIG. 18. As can be seen from the drawing, a leap in the collector-to-emitter voltage Vce when turning off (a switching surge) and a leap in the collector current Ic when turning on occur in the power conversion device. Further, when the switching surge exceeds the breakdown voltage of the semiconductor switching element, it may lead to element destruction, which has a considerable effect on the reliability of the power conversion device.
Also, the sharp changes in voltage and current caused when switching, and a resonance phenomenon occurring in accompaniment to the sharp changes, cause a high level of noise, resulting in conduction noise conducted to the power supply system, and radiation noise propagated to a space on the periphery of the power conversion device and a cable connected to the device. There is a demand to reduce the conduction noise and radiation noise so as not to cause peripheral instruments to malfunction. For example, regulation values of 150 kHz to 30 MHz for conduction noise and 30 MHz to 1 GHz for radiation noise are stipulated by the International Electrotechnical Commission (IEC).
A power conversion device shown in FIG. 19 is a schematic circuit diagram showing a main portion of a motor drive circuit. The motor drive circuit shown in the diagram is such that a converter 2, which converts alternating current voltage provided from a 3-phase alternating current power supply 1 into direct current voltage and outputs the direct current voltage, and a smoothing capacitor Cdc, which stabilizes the direct current voltage output from the converter 2, are connected between a positive line 6 and a negative line 7, which are direct current power supply lines. Furthermore, the motor drive circuit includes an inverter 3, which receives the direct current voltage stabilized by the smoothing capacitor Cdc and outputs a 3-phase alternating current voltage of an arbitrary frequency. Further, the 3-phase alternating current voltage output from the inverter 3 is provided to a motor M, whereby a desired rotation speed is obtained.
Specifically, the converter 2 is such that three series circuits (D1 and D4, D2 and D5, and D3 and D6), wherein two rectifier diodes are connected in series, are connected in parallel, and alternating current voltage provided from the 3-phase alternating current power supply 1 is converted to direct current voltage. Also, the inverter 3 is configured of three series circuits (S1 and S4, S2 and S5, and S3 and S6), wherein two IGBTs are connected in series, connected in parallel. The inverter may also be configured using, for example, a so-called 2-in-1 type power module wherein two switching elements are connected in series, a 6-in-1 type power module wherein six switching elements are connected in a bridge, or a PIM (Power Integrated Module) wherein a converter circuit wherein six rectifier diodes are connected in a bridge and an inverter circuit wherein six switching elements are connected in a bridge are packaged. Also, the inverter 3 is, for example, PWM controlled by an unshown control circuit.
Herein, Lp1, Lp2, Lp3, Ldcp, Ldcn, Ln1, Ln2, and Ln3 in FIG. 19 are line inductors that exist in printed patterns or bus bars configuring the direct current power supply lines of the converter and inverter, and are a main cause of switching surge occurring. Although the line inductors Lp1, Lp2, Lp3, Ldcp, Ldcn, Ln1, Ln2, and Ln3 are not depicted in a normal circuit diagram, they exist due to the structure of the heretofore described power conversion device and the like.
Further, the larger the values of the line inductors, the larger the switching surge becomes. This is because, when the IGBTs (S1 to S6) are turned off in the circuit of FIG. 19, current flowing through the line inductors loses a conductive path.
As a general countermeasure for suppressing this kind of switching surge, there is a method whereby a snubber circuit is connected. The snubber circuit performs a role of absorbing energy accumulated in the line inductors, thereby suppressing the switching surge. FIG. 20 is such that a snubber capacitor Cs is connected between the positive line 6 and negative line 7, which are direct current power supply lines, in the power conversion circuit shown in FIG. 19. In the drawing, the line inductors Lp1 and Ln1 are included in a printed pattern or bus bar between the smoothing capacitor Cdc and snubber capacitor Cs.
Line inductors Lsp and Lsn, in which the inductance of the snubber capacitor Cs's own lead and the inductance of a connected printed pattern or bus bar are combined, are included in the snubber capacitor Cs.
Alternatively, as another general countermeasure for suppressing the switching surge, although not particularly shown in the drawings, there is a countermeasure whereby a snubber circuit, wherein a parallel circuit configured of a diode and resistor is connected in series with a capacitor, is connected in parallel with the switching elements. However, even when adding this kind of snubber circuit, it is often the case that it is difficult to sufficiently reduce conduction noise and radiation noise.
For example, referring to the switching waveforms shown in FIG. 18, it is the peak voltage of the switching surge that is the cause of element destruction when the IGBT is turned off. Meanwhile, it may happen that the switching surge, not converging immediately after the peak value, resonates and oscillates. This resonance phenomenon may also be observed in the current when turning on.
Although this kind of resonance phenomenon is not a cause of element destruction, it is a cause of the occurrence of noise that causes an extremely high value in a frequency spectrum, as shown in FIG. 21, and results in the occurrence of large radiation noise or conduction noise. Also, the main causes of the resonance phenomenon are parasitic capacitance of circuit elements of the snubber circuit, or the like, added in order to suppress switching surge, and of the switching elements themselves, and resonance caused by line inductance or the like in the periphery of the switching elements. A method whereby a series circuit wherein a capacitor and resistor are connected in series is connected in parallel with a direct current power supply line having line inductance has been tried as a method of reducing high level conduction noise or radiation noise occurring due to this kind of resonance phenomenon, for example, as disclosed in Japanese patent document JP-A-2010-41790.
However, a power conversion device described in JP-A-2010-41790 is such that there is also parasitic inductance in the capacitor connected in parallel to the direct current power supply line, and there is concern that the value of that inductance is generally greater than the inductance value of the direct current power supply line with which the capacitor is connected in parallel. At this time, series resonance due to the capacitor and the parasitic inductance of the capacitor occurs at a frequency lower than a frequency at which it is hoped that parallel resonance of the direct current power supply line and capacitor will occur. Therefore, as the capacitor acts as parasitic inductance at the parallel resonance frequency, the envisaged parallel resonance of the capacitor and direct current power supply line does not occur. Because of this, it is difficult to reduce the high level conduction noise and radiation noise.